Vapor pressure lowering occurs when a non-volatile solute is added to a solvent, resulting in a decrease in the solvent's vapor pressure. This happens because the solute particles occupy space at the surface of the liquid, reducing the number of solvent molecules that can escape into the vapor phase. This phenomenon is a key concept in colligative properties, which are properties that depend on the number of solute particles in a solution rather than their identity. Vapor pressure lowering is important in various applications, including distillation and freezing point depression.